1. Field of the Invention
This invention relates generally to image covertor cameras, and more particularly to electric streak cameras capable of measuring, for instance, temporal variation of intensity of a light emission phenomenon which varies at high speed.
2. Description of the Prior Art
The general arrangement of a conventional streak camera will be briefly described.
FIG. 6 is a sectional view of the conventional streak camera. A slit S1 is formed in the front end of a dark box B so that it extends perpendicular to the surface of the drawing. The image of a light beam to be measured, which has passed through the slit S1, is applied as a linear image to the photocathode Pd of the streak tube S through a lens L.sub.1.
The streak tube S comprises the aforementioned photocathode Pd; a mesh electrode Me; a focusing electrode F; an aperture plate A; a deflecting electrode D; a microchannel plate M; and a phosphor screen Ph.
The electrons produced by the photocathode Pd are accelerated by the mesh electrode Me, focused by the focusing electrode F, and sent through the aperture A into the deflecting space defined by the deflecting electrode D, where the electrons thus treated are deflected.
The number of electrons thus deflected is multiplied by the micro-channel plate M and applied to the phosphor screen Ph. As a result, a streak image is formed on the phosphor screen Ph where electron images are aligned in order of its departure time from the photocathode; a time axis as a vertical axis. The image thus formed is projected by a lens L.sub.2 onto the photocathode of an image pickup device which composes a streak image analyzing device.
The streak camera has variety of applications as it can record or measure light intensity which varies at high speed with high time resolution.
The time resolution of the streak camera is determined mainly by the characteristics of the streak tube S, the sweeping (or deflecting) speed of the streak camera, and the slit width.
An essential characteristic of the streak tube S is mainly the photoelectron transit time difference caused when photoelectrons travel from the photocathode Pd on the deflecting electrode D.
The time resolution .DELTA.t of the streak camera can be expressed by the following equation: EQU .DELTA.t=((.DELTA.t.sub.1).sup.2 +(w/v).sup.2).sup.1/2
where .DELTA.t.sub.1 is the transit time difference (s) determined by the characteristic of the streak tube S; v is the sweeping speed (m/s) on the phosphor screen Ph; and w is the full-width at half maximum of the slit image on the phosphor screen Ph provided when the sweeping of the streak tube S is suspended.
The typical values of the above data are as follows: EQU .DELTA.t.sub.1 =1.6 ps, w=60 .mu.m, and v=5.times.10.sup.7 m/s.
In this case, the time resolution is approximately 2 ps. EQU .DELTA.t.sub.1 =5 ps, w=120 .mu.m, and v=1.5 .times.10.sup.7 m/s.
In this case, the time resolution is approximately 9.4 ps.
As is apparent from the above description, in order to obtain a time resolution on the order of the above-described values, it is necessary that the full-width at half maximum of the slit image on the phosphor screen Ph be set between 60 and 120 .mu.m.
Therefore, in the case where the image magnification of the streak tube (magnifying the image of the photocathode on the fluorescent surface) is 1.5 to 3, it is necessary to form the slit image with a width of 10 to 30 .mu.m on the photocathode of the streak tube.
As the resolution can be increased by decreasing the value w, a variety of methods of decreasing the width of a light beam applied to the photocathode Pd have been proposed.
In the case of FIG. 6, in order to obtain a narrow slit image on the photocathode, the light beam is passed through the incident slit S1, and the image of the incident slit S1 is formed on the photocathode by means of the relay lens L.sub.1. In the measurement of extremely weak light such as the measurement of the waveform of a laser excitation fluorescence (or a light beam having large solid angle), it is necessary to collect as much light as possible to the incident slit. For this purpose, a method has been proposed in which, as shown in FIG. 7, a cylindrical lens L.sub.2 is arranged in front of the slit S1 of the dark box B. In another method, as shown in FIG. 8, an optical fiber L.sub.3 is used to lead the light beam to be measured to the incident slit S1 (FIG. 6) so that the width of the light beam is limited by the incident slit S1.
The above-described methods can limit the width of the light beam applied to the photocathode. However, these methods still have disadvantages. For example, since the slit is used, it is rather difficult for the streak camera to receive a sufficiently large quantity of input light.
Accordingly, it is an object of this invention to increase the concentration of light received by a streak camera for enhancing the quality of measurement.
Another object of the invention is to control the light beam to be measured in a streak camera in order to provide better resolution of the images produced.
A further object of the invention is to improve the application of a streak camera to a broader range of light intensities.
Additional objects and advantages will be obvious from the description, or may be learned by practice of the invention.